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Swimmers on Monday were coming out of the water covered in a metallic-colored slime. Phil Rogers reports. (Published Tuesday, June 18, 2013)

The slick, slimy substance that forced an evacuation of a northwest Indiana beach on Monday has been identified as a material found in fertilizer.

The Indiana Department of Environmental Management tested the substance that covered dozens of Porter Beach swimmers and determined it's tricalcium orthophosphate. Besides being found in fertilizer, it is sometimes used as a food additive.

An area company manufacturers it, but the IDEM believes it came from a boat or a barge in the lake.

IDEM spokesman Dan Goldblatt said officials also found traces of another material called digluconic acid, which is a metal cleaner.

Goldblatt said the U.S. Coast Guard has been tasked with trying to find the source of the substance. The agency is checking vessels that might have made the passage in the last 48 hours.

In the meantime, Ken Mehne with the National Park Service says they are telling people it is now safe to go back in the water, though the lake is very cold.

Mehne says he has been led to believe that the concentrations that were in the water on Monday, even when the material was visible and obvious, were not hazardous.

Officials from several other agencies, including the National Park Service, the U.S. Geological Survey, and the coast guard took samples of the water near Porter Beach. Though swimmers were ordered out of Lake Michigan, Porter Beach and nearby Indiana Dunes remained opened.

"I’ve been here eight years and I’ve never seen this," he said. "Never had to close the beach because of some type of substance."

Described as a "slight discoloration" but not a different texture, the affected area was estimated to be about 50 feet long by 50 feet wide. It was about 150 feet out into the water and was dissipating as it moved from west to east in the water, said Baughman.

Editor's Note: A previous version of this article carried a lead image showing a brown substance flowing down a beach into the waters of Lake Michigan at Dunes Creek. Our photographer snapped this photo with a still camera while capturing video of the waters off-shore. While visually concerning, a water resource specialist with the Save the Dunes organization explained that what is seen at Dunes Creek is a natural discoloration of the water from its passage through the Black Oak stands of the nearby forest.

It is not in any way associated with the slimy substance that some swimmers encountered nearby on Monday. NBC Chicago regrets the error.